HR: 1340h
AN: OS43A-0533    [Abstracts]
TI: Finding a Paleoproxy for Upper Ocean Nitrate Concentrations: Se/S in Marine Barite?
AU: * Cutter, G A
EM: gcutter@odu.edu
AF: Dept. of Ocean, Earth and Atmospheric Sciences, Old Dominion University, 4600 Elkhorn Ave., Norfolk, VA 23529-0276 United States
AU: Cutter, L S
EM: lcutter@odu.edu
AF: Dept. of Ocean, Earth and Atmospheric Sciences, Old Dominion University, 4600 Elkhorn Ave., Norfolk, VA 23529-0276 United States
AU: Paytan, A
EM: apaytan@pangea.stanford.edu
AF: Dept. of Geological and Environmental Sciences, Stanford University, Braun Hall 207, Stanford, CA 94305-2115 United States
AB: Nitrate is a key limiting nutrient in many oceanic environments and a paleoproxy for nitrate would be invaluable for understanding mechanisms driving C export fluxes in the past. Selenate (SeO$_{4}$$^{2-}$), one of the dissolved chemical species of the trace element selenium, shows an excellent correlation with nitrate, having surface water depletion and deep water enrichment. In this respect, the biogeochemical cycle of selenium is analogous to that of nitrogen. Furthermore, selenate is almost chemically identical to sulfate and as such is incorporated into barite, a diagenetically stable mineral that is formed inorganically in the water column. In order to investigate whether the Se/S ratio in marine barite can act as a nitrate proxy, we have utilized a set of core top samples from $8\deg$N to $5\deg$S at $140\deg$W in the Equatorial Pacific Ocean, as well as sediment samples from other ocean basins. Barite was isolated from these sediments using sequential leaching and ashing, and then dissolved using a cation chelation method. Dissolved sulfate in the resulting solutions was determined by ion chromatography, while selective hydride generation-atomic absorption spectrometry was used to determine dissolved selenate. A N-S transect at $160\deg$W in the Pacific showed both dissolved selenate and nitrate to be depleted in surface waters of the northern and southern gyres, while concentrations rose to 0.6 nmol Se/L and 3.5 $\mu$mol N/L in the equatorial upwelling regime (the r for Se vs. N was 0.92). In comparison, barite Se/S in surficial sediments underlying oligotrophic waters averaged 11.5$\pm$5.8 x 10$^{-8}$, while those under the equatorial upwelling regime were 92$\pm$64 x 10$^{-8}$, values that are ca. 10-100x those in surface waters (Se enrichment in barite relative to S). These results suggest that barite may indeed be recording the upper ocean selenate, and hence nitrate, concentrations. However, more data are required and experiments to quantify the amount of Se enrichment relative to S in barite (vs. in surface waters) need to be conducted.
DE: 4805 Biogeochemical cycles (1615)
DE: 4825 Geochemistry
DE: 3035 Midocean ridge processes
DE: 1030 Geochemical cycles (0330)
DE: 1050 Marine geochemistry (4835, 4850)
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting